1. Field of the Invention
The invention relates to a fuel cell device.
2. Description of Related Art
Conventionally, a cell stack including a plurality of stacked unit cells is used in a fuel cell. The fuel cell including the cell stack is accommodated in a case in a state in which a load is applied to the cell stack in a stacking direction in order to maintain the stack structure of the cell stack. Japanese Patent Application Publication No. 2013-012325 (JP 2013-012325 A) describes a fuel cell device that includes a tension shaft disposed along a bottom surface of a cell stack in a state in which a fuel cell is accommodated in a case. One end of the tension shaft is fixed to a case side surface opposing an end surface of the cell stack in a stacking direction and the other end thereof is fixed to an end plate positioned on the opposite side of the cell stack from the above-mentioned case side surface, and the case, the cell stack, and the end plate are thereby fastened to each other.
In the fuel cell device described in JP 2013-012325 A, the case and the tension shaft are formed of a metal material of the same type such as steel. In addition, since the tension shaft is a rod member, the cross-sectional area of the tension shaft along a direction perpendicular to the stacking direction is smaller than the cross-sectional area of the case along the direction perpendicular to the stacking direction. Consequently, the rigidity of the tension shaft is lower than the rigidity of the case in the direction perpendicular to the stacking direction. Incidentally, the temperature of the cell stack increases during power generation, and the cell stack thermally expands and a compressive load increases. Accordingly, in the fuel cell device described in JP 2013-012325 A, with the increase of the compressive load, the dimensional change of the tension shaft having lower rigidity becomes larger than the dimensional change of the case having higher rigidity, and a fastening force on the side of the tension shaft (the side of the bottom surface) is reduced in the cell stack. With this, the side of the bottom surface of the cell stack expands in the stacking direction as compared with the side of an upper surface of the cell stack, and parallelism between unit cells is reduced. Thus, when the parallelism between the unit cells is reduced, surface pressures of the unit cells become uneven, and hence there is a possibility that power generation performance of the fuel cell device is lowered. To cope with this, a technique capable of preventing the reduction in parallelism between the unit cells when the temperature of the fuel cell device increases during power generation is desired.